Up north, since the front door is in another room, I simply don't answer it.
It's not terribly uncommon for burglars to knock to determine if anyone is home and if they get no answer, to attempt entry.
Awhile back, I put up a No Solicitors sign--not because I think it will keep burglars away but because it reduces the incidence of people knocking on the door tremendously. Now it happens so rarely that 1.) it's not a major hassle to at least get up and check it out. and 2.) I don't get too complacent about it.